An LED drive circuit is known, which applies a voltage obtained by full-wave rectifying a commercial alternating-current power source (hereinafter, referred to as a full-wave rectified waveform) to an LED string and turns on the LED string. In many cases, a current limiting circuit in the LED drive circuit is connected in series with the LED string in order to stabilize the operation (for example, see FIG. 3 of Patent Literature 1).
FIG. 28 is a circuit diagram of a conventional LED drive circuit 10 (LED lamp) illustrated in FIG. 3 of Patent Literature 1. In FIG. 28, symbols are changed from those of Patent Literature 1. In the following explanation, the term of Patent Literature 1 is described within brackets.
The LED drive circuit 100 (LED lamp) illustrated in FIG. 28 includes a bridge rectifier circuit 106 (rectifier circuit), a zener diode 115 (constant voltage diode), a constant current diode 116, and a plurality of LEDs (Light Emitting Diodes) 101a to 101n. A commercial alternating-current power source, not illustrated schematically, is connected to input terminals 111 and 113 of the bridge rectifier circuit 106. The zener diode 115, the constant current diode 116, and the plurality of LEDs 101a to 101n are connected in series. The cathode of the zener diode 115 is connected to a terminal that outputs a current of the bridge rectifier circuit 106 and the cathode of the LED 101n is connected to a terminal to which the current of the bridge rectifier circuit 106 returns.
The bridge rectifier circuit 106 outputs a full-wave rectified waveform from the terminal that outputs a current when the terminal to which the current returns is grounded. The zener diode 115 is used as a voltage drop element. The constant current diode 116 corresponds to the current limiting circuit that is connected in series to the LED string described previously. The plurality of LEDs 101a to 101n is connected in series and forms an LED string.
In the LED drive circuit 110 illustrated in FIG. 28, when the full-wave rectified waveform is in a low voltage phase, no current flows through the LED string, and therefore the LEDs 101a to 101n do not turn on. When the voltage of the full-wave rectified waveform rises and the voltage between both ends of the LED string exceeds a threshold voltage of the LED string, a current flows through the LED string, and therefore the LEDs 101a to 101n turn on. At this time, the upper limit value of the current that flows through the LED string is limited by the constant current diode 116, and therefore the current is constant during almost all the period in which the LEDs 101a to 101n are on. The threshold voltage of the LED string is a sum of the forward voltage drop of each of the LEDs 101a to 101n connected in series within the LED string.